Magnetic resonance imaging (MRI) is an invaluable tool in clinical imaging. MRI uses a strong static magnetic field (B0), a powerful radio-frequency (RF) field (B1), and rapidly switching magnetic field gradients to generate and spatially encode signals. Due to these magnetic fields, patient monitoring during the MRI procedure can be challenging, as it is difficult to operate electronics in this environment. The static fields used are often between 1.5 T and 7 T, which precludes the use of ferromagnetic materials in any electronic device placed inside the scanner bore. The RF field is a source of electromagnetic interference and the gradient fields are a source of mechanical vibrations and eddy currents in conductive materials. Not only must any electronic monitoring device be capable of operating in this environment, but it should not perturb the MRI fields in any way, as this can result in artifacts in the MR images.
Despite these challenges, it is advantageous to be able to optically monitor the head or body of the patient during the MRI procedure. One major area of application is the correction of motion. MRI is extremely sensitive to motion. Head movements of only a few millimeters during a typical 5-10 minute scan produce severe image artifacts, often rendering the images useless. This can affect the outcome for the patient and increases costs if scans must be repeated. One promising motion correction method involves measuring the head pose (position and orientation) of the subject using optical camera systems. Video information is used to track a marker mounted on the head of the subject. Head motion data are then used for real-time control of the scanner. This involves updating the RF and gradient fields of the scanner to compensate for the motion of the head, thereby ensuring that there is no relative motion between the imaging volume and the object. This technique is very powerful, as it is applicable to all common imaging sequences.
Motion correction has motivated several recent developments in patient monitoring for MRI. One existing approach involves placing cameras outside the scanner away from the strongest magnetic fields (U.S. Pat. No. 8,121,361). This has the major disadvantage that an unimpeded line of sight to the tracking marker attached to the subject is required. Another implementation involves placing cameras inside the bore of the MRI scanner, either directly attached to the bore or mounted on the head coil used to receive RF signal (US 2009/0209846 and US 2011/0201916). This helps achieve clear line of sight, although parts of the head coil can still obscure the view of any optical marker used. To circumvent the problem of obscured view to an optical marker, a self-encoded marker can be used that allows determination of which part of the marker is seen by the camera (US 2012/0121124). Furthermore, the need to provide power to the apparatus via a conductive cable can cause electromagnetic interference with the MRI procedure. Finally, existing implementations require cameras to be manually positioned and calibrated by the scanner operator, in some cases prior to every scan. These issues have been a barrier to the adoption of the technique as a routine clinical tool because they interfere with workflow.
In addition to motion correction, there are many other uses for high-quality video information from the subject during the MRI procedure. Eye tracking is often used in functional MRI (fMRI) experiments; patient skin temperature could be measured by thermal mapping leveraging the thermal sensitivity (e.g. infrared) of an optical detector; physiological signals, such as pulse rate and oxygen saturation, could be detected from slight color changes in the skin; respiratory signals could be measured from optical motion data without requiring the use of a pneumatic respiratory belt. It would therefore be an advance in the art to provide an optical imaging system that (a) ensures unimpeded line of sight to the subject (b) is completely MR compatible and (c) requires minimal user interaction to setup and maintain.